Generally, it is known that the luminescence intensity of semiconductor light emitting element such as light emitting diode varies according to elapsed time or temperature variation. For example, as for elapsed time, it is known that the luminescence intensity decreases according to deterioration of semiconductor light emitting element. In the case of APC driving or constant light output driving, a drive current or a drive voltage increases according to deterioration of semiconductor light emitting element, as a result, the element eventually cannot emit light, and its life will be over. In addition, in a semiconductor laser diode (LD) or the like, it is known that, when the temperature rises, its threshold current increases and a required drive current or drive voltage increases to provide the same light emission output in some cases. Similarly, in a light emitting diode, it is known that, when the temperature is high, in the case of APC driving i.e., constant light output driving, or the like, its light emission output decreases. On the other hand, when the temperature is low, even in the case of the same current, a larger amount of light emission is obtained.
If fluctuation or variation of light emission output of semiconductor light emitting element according to elapsed time or temperature variation arises, it is difficult to achieve construction of precise measurement system, construction of highly reliable communication equipment, and so on, in optical fiber communication system. In the case of display or lighting composed of light emitting diodes, they may cause unevenness of light intensity or color. For this reason, conventionally, a circuit that is provided with a light output controller 500 to provide temperature compensation for the fluctuation variation of light emission output as shown in FIG. 1 has been devised. In brief description of FIG. 1, the light emission output of a light emitting element 100 varies according to temperature. The light emission output is proportional to a drive current. Accordingly, for example, in the case where the light emission output increases according to temperature variation, the light emission controller 500 serves to reduce a current running through the light emitting element 100. On the other hand, control is performed such that a current running through a field-effect transistor 200 is constant, thus, a bypass current runs through the light emission controller 500. As a result, the light output is constant.
In the other case where the light emission output decreases according to temperature variation, the light emission controller 500 serves to increase a current running through the light emitting element 100 by reducing a bypass current running through the light emission controller 500. As a result, the light output is constant. In the light emission controller 500, a circuit is composed of a FET, a bipolar transistor, etc., and a thermistor. A thermistor is a variable resistor with temperature dependence. Accordingly, a constant-current circuit with temperature dependence is constructed by using a thermistor to provide a stabilized light source with less fluctuation according to elapsed time or temperature variation. In addition, instead of a variable resistor such as thermistor, a voltage generation circuit that has a normal resistor and a silicon diode with a temperature coefficient (e.g., −2 mV/° C. in forward voltage) so as to reduce a bias voltage as temperature rises is constructed to be used in an integrated circuit for a semiconductor light emitting diode or semiconductor laser diode.
Although the case where one semiconductor light emitting element is used alone or a monochromatic semiconductor light emitting element is used is discussed above, the case of a lighting apparatus or display that employs a plurality of combined light emitting elements is similar. That is, for example, in a RGB white LED device composed of red, blue and green LEDs, for fluctuation of light emission output according to elapsed time or temperature variation that affects each LED, a temperature compensation circuit or the like with thermistor, etc., is constructed each, as mentioned above. Alternatively, red, blue and green sensors are provided to constantly measure and monitor respective luminescence intensities of RGB wavelengths, respectively, the luminescence intensities are fed back to respective drive circuits for the RGB LEDs for control so as to bring the respective luminescence intensities of RGB wavelengths desired constant values irrespective of temperature variation, elapsed time, deterioration, and so on. This type of construction is used.                Patent Document 1: Japanese Laid-Open Patent Publication TOKUKAI No. HEI 4-196368        Patent Document 2: Japanese Laid-Open Patent Publication TOKUKAI No. SHO 64-48472        
However, conventionally, an object to be controlled by temperature compensation is a luminescent intensity. That is, in lighting, or the like, that is composed of a plurality of semiconductor elements with different wavelength and has a predetermined chromaticity such as white light, in the case where the temperature fluctuates, or the like, conventional temperature compensation for luminescent intensity cannot compensate shift or fluctuation of wavelength of each semiconductor light emitting element such as LED. As a result, there is a problem where the chromaticity of the white lighting, or the like, composed of semiconductors that have shifted (or fluctuated) wavelengths shifts from an initial chromaticity before their wavelengths shift (or fluctuates).
In other words, for example, an LED device composed of RGB three-wavelength light emitting diodes, even in the case where drive control is performed by a feedback circuit with a sensor, or the like, provided therein such that respective light emission intensities of the respective colors of light emitting diodes are kept constant, as shown in FIG. 2, as it is known that the chromaticity (or wavelength property) of light emitting diode fluctuates, even if respective luminescent intensities of the RGB light emitting diodes having wavelength properties or chromaticities that shift from initial drive, as shown in FIG. 3, it is impossible to maintain a predetermined chromaticity in the initial drive are kept constant. Even if the chromaticity is still in white, the obtained white output light has a tint that subtly fluctuates toward reddish side or greenish side. That is, as shown in a schematic x-y chromaticity diagram of FIG. 3, although the color of the RGB LEDs in the initial drive can show the triangle region shown by a solid line in the figure, even if adjustment of the luminescent intensities of RGB light emitting diodes sets the chromaticity at “initial white” shown by a solid circle in the figure, when the temperature fluctuates, chromaticities of RGB also fluctuates to R′G′B′ as shown by arrows. In this case, even if the light outputs of the RGB colors of light emitting diodes are kept constant irrespective of temperature fluctuation, subtle fluctuation of wavelength properties, i.e., chromaticities of colors shown in FIG. 2 causes fluctuation from the initial RGB solid-line triangle to a R′G′B′ dashed-line triangle. For this reason, maintenance of luminescent intensity to the same luminescent intensity in the initial drive cannot maintain the chromaticity in the initial drive, in this case, “initial white”. Similarly, fluctuation occurs according to a drive current value as shown in FIG. 2(b). The wavelength property fluctuates according to fluctuation of a drive current value. That is, chromaticity fluctuation phenomenon occurs in the case of light emitting element, and so on. Particularly, as for semiconductor light emitting elements, in some cases of materials or structures, wavelength shift or the like due to deterioration or temperature fluctuates. On the other hand, it is conceivable that light from a light emitting apparatus is directly sensed by a photo sensor, and thus is corrected for color shift, and so on. In order to perform correction with a sensor, for example, it is conceivable that, in consideration of a variation amount of light passing through each filter of RGB as color shift, adjustment to a desired color tone, or the like, is performed by controller that receives feedback of light amount of light emitting element. However, in this case, it is very difficult to provide fine adjustment of the chromaticity depending on the color filter property. If the numbers of filters and sensors are increased, it is possible to provide fine adjustment. But, this causes device complexity and high cost, and thus provides trade-off.